The invention relates to a Penning sputter source comprising an anode, a cathode which is separated from the anode and forms a target, an exciting magnet, a system for cooling the target and a screen system which preferably forms the outer housing of the sputter source. The proposed Penning sputter source can be preferably used in the production of semiconductor thin films and assures uniform sputtering with a good effectiveness.
According to the present level of technology in the electronics industry two kinds of vacuum methods are used for producing thin films, namely the vaporizing and sputtering methods.
Up to the beginning of the 1970's the vaporizing methods, particularly the electronbeam vaporizing techniques had a general acceptance. These methods are very effective in producing monolithic semiconductor thin films having a homogenous structure and formed on thermostable substrates. The disadvantage of these methods lies in the great temperature used for vaporization. Different metals have different partial pressures at the temperature of vaporization which renders the production of alloy layers difficult. During the vaporization a great quantity of heat is released; therefore the use of light plastic substrates is limited. The impact of electron beams on the vaporized material results in X-ray radiation and this radiation can spoil the characteristics of the sensitive semiconductor MOS structures.
The sputtering methods are more advantegous in producing alloy layers. A further advantage of these methods is the possibility of producing layers on plastic substrates. The sputtered thin films are of better quality than the vaporized ones and their production technology is preferable in comparison with the vaporization methods.
The first sputtering methods were based on the well-known cathode sputtering effect which occurs in electron tube structures containing diodes or triodes. These methods show a low speed and a low effectiveness of production. Their main advantages are small quantity of released heat, the absence of electron beams and of their associated X-ray radiation, and their great scattering (the sputtered material is scattered in a large beam, therefore good quality step coverages can be made with a uniform thickness of the layer).
New sputtering methods and devices were developed on the basis of field emission for increasing the speed and the effectiveness of the technological processes. The basic idea of these methods and devices is the exploitation of the Penning gas discharge in a vacuum, which by means thereof the effectiveness of the field emission may be sufficiently increased.
According to the Penning methods of plasma generation, a cathode to be sputtered and an anode are situated in a preliminary vacuum space comprising discharge gas, generally noble gas, suitably argon. The anode and the cathode are exposed to a stable magnetic field and connected to a direct current supply unit giving a direct current of a value preferably within the range of 400 and 800 V. In this way, an electric field is generated around the device being situated in the stable magnetic field as mentioned above. The discharge gas, whose atoms thereof are ionized for sputtering the material of the cathode, can be composed also of reactive gases. In this case chemical compounds also can be condensed on the carrier plate, for example, for producing isolating layers. The magnetic field is directed parallel to the surface of the cathode and has a value being greater then a threshold value which depends on the material of the cathode and on other conditions. In a preliminary vacuum within the range of 0.1 and 1 Pa the suitable range of the magnetic field intensity is of 0.01 and 0.025 T (tesla). This magnetic field gives a remarkable increase of the intensity of the field emission.
The anode and the cathode, as mentioned above, are connected to a direct current supply unit giving a potential difference of required value. For generating the magnetic field a permanent magnet or an electromagnet is used. In the magnetic field the electrons move on a spiral trajectory therefore traversing a longer distance than the distance between the cathode and the anode. The probability of the ionization of the atoms in the discharge gas increases with the length of the trajectory of the electrons. In the magnetic field the ionized atoms are impacted onto the surface of the cathode which forms the target. The target is sputtered in this way by the ionized atoms being accelerated by the electric field. The sputtered material is condensed on special substrates and forms a semiconductor thin film.
The optimal conditions of the Penning plasma generator can be obtained by application of magnetic and electric fields perpendicular to each other. The gas discharge also can be initiated in a high vacuum (this is the basic idea of the well-known Penning vacuum meter).
The Penning gas discharge being initiated in the space of a preliminary vacuum assures the flow of a gas discharge current which sometimes has an intensity an order of magnitude greater then the intensity of the same current in absence of the magnetic field. The speed of sputtering depends mainly on the current intensity; therefore the Penning discharge guarantees a speed of sputtering at least one order of magnitude greater then the speed of the afore-known sputtering processes.
The known Penning sputter sources contain a target which is connected as cathode, a cooling system, a field-magnet or exciting magnet, an anode which is separated from the cathode, and a screen system which preferably forms an outer housing of the device. The screen system may be formed for example from copper. The target must be cooled owing to the unavoidable heating effect of the current supplying the sputter source, such that the water cooling is generally used. The magnetic field is generated by a permanent magnet or by an electromagnet which is placed under and/or around the target. The target of the sputter source can be made for example in the form of a circle, a rectangle with broken corners, a racetrack consisting of two parallel direct lines and two half-circles being connected therewith, or even in the form of a triangle with broken corners. The sputtered area of the target forms the so-called active zone where the sputtering ions are impacted. The active zone can be produced from high-priced materials forming a thin upper layer of the target, while the rest of the target represents a support for the active zone.
The Penning sputter sources are available under different names, as S-gun, Planar Plasmatron, Planar Magnetron, Orbitorr etc.
The simplest types of the known Penning sources comprise a plain, generally disc-like target surrounded by a screen system which forms an anode. Under the target a permanent magnet is placed for generating a stable magnetic field. In this simple structure the conditions of plasma generating can be guaranteed only over a small surface area of the target. Therefore the active zone is of extremely limited dimensions. In this device, the distribution of the magnetic field is inhomogenous over the target; therefore the material to be sputtered is utilized unevenly, and the sputtering process remains of limited expansion.
Owing to the afore-mentioned disadvantages, a new-type Penning sputter source was constructed by the Varian Corp. (U.S.). This source is known under the trade name of S-gun and is described in detail by Vance and Hoffmann in "S-gun--a new sputtering system" (Varian Seminar, Palo Alto, 1978). This device is based on the use of a ring-like exciting magnet surrounding a conical target. The lines of the magnetic field are parallel to the conical surface of the target for a greater area then in the devices with a disc-like targets and this provides for an increase of the surface area of the active zone.
The construction of the target in the above device is ingenious. The target has dimensions sufficient to situate it in thermal contact with the surrounding cooling system. When the sputtering current results in an unavoidable thermal expansion of the target the thermal conduction to the cooling system is increased. In this device, the active zone has a greater surface and the conditions of the plasma generator are better then in the afore-known devices. A further new element of the S-gun type source lies in the central situation of the anode in comparison with the active zone.
The magnetic field is utilized in the S-gun type Penning sputter source with a low effectiveness because the exciting magnet is situated relatively far from the active zone. For increasing the intensity of the magnetic field the geometrical dimensions of the device must be increased. The conical form of the target can be a factor causing technological difficulties during its production, particularly in the case when different materials are used for the active zone and for the rest of the target.
All types of the known Penning sputter sources can be characterized by inhomogenous conditions of the plasma generator over the target and therefore by an unequal and disadvantageous erosion of the active zone during the sputtering process.